Finding of the same antigens in the polychaete, Pseudopotamilla occelata, as those in the vanadium-rich ascidian, Ascidia sydneiensis samea

From sea squirts to squirrelfish: facultative trace element hyperaccumulation in animals

The hyperaccumulation of trace elements is a widely characterized phenomenon in plants, bacteria, and fungi, but has received little attention in animals. However, there are numerous examples of animals that specifically and facultatively accumulate trace elements in the absence of elevated environmental concentrations. Metal hyperaccumulating animals are usually marine invertebrates, likely owing to environmental (e.g. constant exposure via the water) and physiological (e.g. osmoconforming and reduced integument permeability) factors. However, there are examples of terrestrial animals (insect larvae) and marine vertebrates (e.g. squirrelfish) that accumulate high body and/or tissue metal burdens. This review examines examples of animal hyperaccumulation of the elements arsenic, copper, iron, titanium, vanadium and zinc, describing mechanisms by which accumulation occurs and, where possible, hypothesizing functional roles. Groups such as the ascidians (sea squirts), molluscs (gastropods, bivalves and cephalopods) and polychaete annelids feature prominently as animals with hyperaccumulating capacity. Many of these species are potential model organisms offering insight into fundamental processes underlying metal handling, with relevance to human disease and aquatic metal toxicity, and some offer promise in applied fields such as bioremediation.

Vanadium-Binding Ability of Nucleoside Diphosphate Kinase from the Vanadium-Rich Fan Worm, Pseudopotamilla occelata

Polychaete fan worms and ascidians accumulate high levels of vanadium ions. Several vanadiumbinding proteins, known as vanabins, have been found in ascidians. However, no vanadium-binding factors have been isolated from the fan worm. In the present study, we sought to identify vanadiumbinding proteins in the branchial crown of the fan worm using immobilized metal ion affinity chromatography. A nucleoside diphosphate kinase (NDK) homolog was isolated and determined to be a vanadium-binding protein. Kinase activity of the NDK homologue, PoNDK, was suppressed by the addition of V(IV), but was unaffected by V(V). The effect of V(IV) on PoNDK precedes its activation by Mg(II). This is the first report to describe the relationship between NDK and V(IV). PoNDK is located in the epidermis of the branchial crown, and its distribution is very similar to that of vanadium. These results suggest that PoNDK is associated with vanadium accumulation and metabolism in P. occelata.

Hyperaccumulation of vanadium in the Antarctic polychaete Perkinsiana littoralis as a natural chemical defense against predation

BACKGROUND, AIM, AND SCOPE

Exceptionally high levels of trace metals have been reported in specific tissues of certain polychaetes. In the present study, the Antarctic fan worm Perkinsiana littoralis was shown to hyperaccumulate vanadium in the branchial tissues, and the hypothesis of an antipredatory strategy has been investigated.

MATERIALS AND METHODS

Trace metals (Ag, Al, As, Ba, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, V, Zn) were measured by atomic absorption spectrophotometry in tissues of P. littoralis and, only for V, in two Antarctic bivalves and in various Mediterranean polychaetes. Subcellular distribution of vanadium was investigated in P. littoralis after differential centrifugations; feeding trials with the Antarctic rock cod Trematomus berancchii were performed to test the palatability of P. littoralis.

RESULTS AND DISCUSSION

Analyses of trace metals in tissues of P. littoralis confirmed the naturally high bioavailability of cadmium due to upwelling phenomena in the investigated area and revealed extremely high concentrations of vanadium up to 10,000 microg/g, in the branchial crowns; much lower concentrations were measured in the body portions and even less in the Antarctic bivalves and in Mediterranean polychaetes. The subcellular distribution indicated that this metal was associated in branchial crowns with both heavy components and vanadium binding proteins; the latter predominated in body tissues, although with a different pattern of molecular weight. Feeding trials suggested that the elevated levels of vanadium in branchial crown of P. littoralis act as chemical deterrents against predation in more exposed tissues.

RECOMMENDATIONS AND PERSPECTIVES

The hyperaccumulation of toxic metals might represent a common antipredatory strategy for unpalatable branchial crowns of sabellid polychaetes, as recently hypothesized also for the high concentrations of arsenic in the Mediterranean Sabella spallanzanii. The evolution of such adaptation and the reasons behind the possibility for different species to accumulate different metals represent a stimulating field of investigation for future studies.

Novel vanadium-binding proteins (vanabins) identified in cDNA libraries and the genome of the ascidian Ciona intestinalis

Ascidians, especially those belonging to the suborder Phlebobranchia, can accumulate high levels of vanadium. Vanadium-binding proteins (vanabins) were first isolated from a vanadium-accumulating ascidian, Ascidia sydneiensis samea, and then the vanabins were cloned, their expression was studied, and metal-binding assays were conducted. In order to unravel the mechanism of vanadium accumulation, we searched for vanabin-like genes in other animals, including other ascidians. A database search revealed five groups of cDNAs that encoded vanabin-like proteins in another ascidian, Ciona intestinalis. The genes encoding C. intestinalis vanabins, CiVanabin1 to CiVanabin5, were clustered in an 8.4-kb genomic region. The direction of transcription of each gene was identical and each gene had a single intron. All the C. intestinalis vanabins were cysteine-rich, and the repetitive pattern of cysteines closely resembled that of A. sydneiensis samea vanabins. Using immobilized metal ion affinity chromatography (IMAC), we found that a recombinant protein of at least one of the C. intestinalis vanabins (CiVanabin5) bound to vanadium(IV) ions.

Vanadium-binding proteins (vanabins) from a vanadium-rich ascidian Ascidia sydneiensis samea

Since the beginning of the last century, it has been known that ascidians accumulate high levels of a transition metal, vanadium, in their blood cells, although the mechanism for this curious biological function remains unknown. Recently, we identified three vanadium-binding proteins (vanabins), previously denoted as vanadium-associated proteins (VAPs) [Zool. Sci. 14 (1997) 37], from the cytoplasm fraction of vanadium-containing blood cells (vanadocytes) of the vanadium-rich ascidian Ascidia sydneiensis samea. Here, we describe the cloning, expression, and analysis of the metal-binding ability of vanabins. Recombinant proteins of two independent but related vanabins, vanabin1 and vanabin2, bound to 10 and 20 vanadium(IV) ions with dissociation constants of 2.1x10(-5) and 2.3x10(-5) M, respectively. The binding of vanadium(IV) to these vanabins was inhibited by the addition of copper(II) ions, but not by magnesium(II) or molybdate(VI) ions. Vanabins are the first proteins reported to show specific binding to vanadium ions; this should provide a clue to resolving the problem regarding the selective accumulation of vanadium in ascidians.

Vanadocytes, cells hold the key to resolving the highly selective accumulation and reduction of vanadium in ascidians

Since Henze discovered vanadium in the blood (or coelomic) cells of an ascidian in 1911, this unusual phenomenon has attracted the interest of many investigators. The highest concentration of vanadium (350 mM) in the blood cells of Ascidia gemmata, which belongs to the suborder Phlebobranchia, is 10(7) times higher than that in seawater. Of the approximately 10 types of blood cells, a combination of cell fractionation and neutron-activation analysis revealed that the signet ring cells were the true vanadocytes. In the vanadocytes, 97.6% of the vanadium is in the +3 oxidation state (III). The extremely low pH of 1.9 found in vanadocytes suggests that protons, concentrated by an H(+)-ATPase, might be linked to the accumulation of vanadium energetically. The antigen recognized by a monoclonal antibody, S4D5, prepared to identify vanadocytes, was determined to be 6-PGDH in the pentose phosphate pathway. NADPH produced in the pentose phosphate pathway in vanadocytes is thought to participate in the reduction of vanadium(V) to vanadium(IV). During embryogenesis, a vanadocyte-specific antigen first appears in the body wall at the same time that significant accumulations of vanadium become apparent. Three different vanadium-associated proteins (VAPs) were extracted from the blood cells of vanadium-rich ascidians. These are 12.5, 15, and 16 kDa in size and are associated with vanadium in an approximate ratio of 1:16. The cDNA encoding the 12.5 and 15 kDa VAPs was isolated and the proteins encoded were found to be novel. Further biochemical and biophysical characterization of the VAPs is in progress.

Identification of a vanadium-associated protein from the vanadium-rich ascidian, Ascidia sydneiensis samea

Ascidians are known to accumulate vanadium in their blood cells (vanadocytes) at extremely high levels which correspond to about 10(6) to 10(7) times the levels of vanadium ions in seawater. The route for the accumulation of vanadium ions from the outside environment into the blood system in ascidians has not yet been discovered. In the present experiments, using a combined technique of anion exchange column and atomic absorption spectrometry, we first extracted a vanadium-associated protein (VAP) from the blood cells of the ascidian Ascidia sydneiensis samea. VAP was estimated to associate with vanadium at an approximate ratio of 1 mol:16 mole. SDS-PAGE and a polyclonal antibody against VAP (anti-VAP) revealed that VAP is composed of at least two types of peptides estimated to be 12.5 kDa and 15 kDa with a minor peptide of 18 kDa and that VAP is localized in the cytoplasm of the vanadocytes.